This invention relates to photographic apparatus and, more particularly, it concerns a photographic system by which photographic film may be properly and uniformly exposed over a range of camera/subject distances using a vaporizable filament flash bulb.
Completely automatic, low-cost photographic cameras in which the photographer need only aim the camera at a subject to be photographed and press a button to achieve proper exposure of camera-contained film, assuming the design limits of the camera and film are not exceeded, are available in the present photographic market from several manufacturing sources. Because the operating range and correspondingly the refinements available in a photographic camera are based largely on the size of the camera lens aperture, low-cost cameras commonly employ relatively small maximum lens apertures on the order of f/11 or smaller so as to ensure a relatively large depth of field, particularly when a fixed focus lens is employed. Film exposure is effected by an electronically controlled shutter system in which shutter speed and aperture size variation are governed by a photoresponsive cell and appropriate circuitry built into the camera. Electronically controlled shutter systems of this type are well-known and sufficiently developed so that the cost of such a system is fully consistent with incorporation in low-cost cameras of the type under discussion.
The relatively small maximum lens aperture of low-cost cameras requires more extensive use of artificial illumination. Again because of cost constraints, at least in terms of the initial investment required to purchase the camera, low-cost cameras are traditionally equipped to receive disposable, vaporizable filament flash bulbs or multi-bulb flash arrays in which illumination energy is fixed. In such cameras, exposure control with flash illumination is often predicated solely on the electronic shutter system together with other fixed design parameters of the camera and of the flash illumination source. In this respect, two basic difficulties regarding control of film exposure are presented; namely, the uniformity of the time interval required for the shutter to close after a photometrically generated close command signal is generated, and the inherent variation in intensity of flash illumination during the flash operation. The result of improper film exposure by flash illumination is referred to as "tracking error" and is manifested by overexposure where the camera/subject distance is less than a design norm or by underexposure where the subject is farther from the camera than the average distance for which the camera and flashlamp are designed.
Heretofore, the problems associated with tracking error using shutter control of flash illumination has been substantially avoided by compensating the circuitry of the electronic shutter control system. The circuitry typically includes a light integrating circuit to control the time interval between opening of the shutter and the command signal for closing of the shutter as a function of the time integral of light intensity received by a photocell, or equivalent, subsequent to the shutter opening. As disclosed in U.S. Pat. No. 3,200,723 issued Aug. 17, 1965 to J. M. Topaz, for example, the circuitry may be modified to shorten the timing interval of the circuit by adding a fixed resistance when flash illumination is used. Basically, the series addition of a fixed resistance to the capacitor and the variably resistive photocell in a battery powered RC timing circuit reduces the time required to reach a trigger voltage (i.e., the shutter closing command signal). The added fixed resistance has the effect of increasing the initial voltage (IR) which increases to the trigger voltage at a rate determined by the photocell resistance which, in turn, varies inversely with the intensity of light thereon. Since the rate of voltage increase is nearly constant, the added fixed resistance used only during a flash exposure mode shortens the effective timing interval of the RC circuit so that initiation of shutter closing is advanced to compensate for the characteristics of flash illumination.
It is to be noted that the aforementioned compensating circuits vary the time only of the closing command signal and that the actual time required for complete shutter closure after this signal remained unchanged. This solution is acceptable where the closing time is sufficiently predictable. However, the latter requires relatively costly shutter design and manufacturing control to minimize the variation in shutter closing time from camera to camera.
With the development of faster film speeds, the problems of tracking error in a flash operational mode are sufficiently more acute that acceptable limits on the tracking error cannot be economically achieved by compensation of the shutter control circuit alone. With more sensitive films, variations in the amount of exposure intensity during shutter closing will have a more noticeable effect on film exposure. Higher film speeds also require less light and if overall cost criteria are to be met, the amount of flash illumination energy supplied in an individual flash bulb should be reduced. Where the total energy of the flash illumination is reduced to accommodate the higher speed films, however, the intensity/time curve of incandescent or vaporizable lamps approaches an isosceles triangle in which light intensity increases at a relatively constant rate during the first half of the time duration of illumination and then decreases at essentially a similar rate during the second half of the time interval. Increased tracking error then occurs as a result of widely varying areas (illumination energy) under the intensity/time curve for different increments of the same time lag (shutter closing time) over the total duration of flash illumination. In other words, the effect of the shutter closing can result in unacceptable overexposure at short camera/subject distances and underexposure at longer than normal camera/subject distances. To reduce this tracking error by increasing the fixed resistance of the anticipation circuit leads to an unacceptably large resistance and also still falls short of providing a complete solution.